Communications system having enhanced fault tolerance

ABSTRACT

The invention relates to communications systems, and particularly to enhancing tolerance to failures in network elements and transmission systems. The fault tolerance is achieved by providing two or more active ATM permanent virtual channel (PVC) connections between a core network element and an access network element, and providing a separate ATM interface unit for each such ATM PVC connection in the core network element. The user and signalling traffic is distributed among these active connections and interface units. In case of a failure in one of the ATM PVC connections or interface units, the communication is maintained over the other connection(s) and interface unit(s). Thus, only part.of the transport capacity is lost, and a total block of communication is avoided. Any new communication will be setup over the other connections.

FIELD OF THE INVENTION

The invention relates to communications systems, and particularly toenhancing tolerance to failures in network elements and transmissionsystems.

BACKGROUND OF THE INVENTION

A mobile communications system generally refers to anytelecommunications system wherein the access point (typically wirelessaccess) to the system can change when users are moving within theservice area of the system. A typical mobile communications system is aPublic Land Mobile Network (PLMN). Often the mobile communicationsnetwork is an access network providing a user with a wireless access toexternal networks, hosts, or services offered by specific serviceproviders.

Currently, third-generation mobile communication systems, such as theUniversal Mobile Communication System (UMTS) and the Future Public LandMobile Telecommunication System (FPLMTS), which was later renamed as theInternational Mobile Telecommunication 2000 (IMT-2000) are underdevelopment. The UMTS is being standardized by the EuropeanTele-communication Standards Institute (ETSI), whereas the InternationalTele-communication Union (ITU) standardizes the IMT-2000 system. Thesefuture systems are basically very similar. For example the UMTS, as allmobile communication systems, provides wireless data transmissionservices to mobile subscribers. The system supports roaming, which meansthat UMTS users can be reached and they can make calls anywhere as longas they are situated within the coverage area of the UMTS.

In the UMTS architecture the UMTS terrestrial radio access network,UTRAN, consists of a set of radio access networks RAN (also called radionetwork subsystem RNS) connected to a core network CN through theinterface lu. Each RAN is responsible for the resources of its set ofcells. For each connection between a mobile station MS and the UTRAN,one RAN is a serving RAN. A RAN consists of a radio network controllerRNC and a plurality of base stations BS. The RNC is responsible for thehandover decisions that require signalling to the MS. The base stationsare connected to the RNC through the lub interface. The base station BScommunicates with the mobile stations MS (or user equipments UE) overthe radio interface Uu. The Uu and lub interfaces are not relevant tothe present invention and will not be described in more detail herein.Further information can be found in the UMTS specifications.

In the interface lu between the radio network controller RNC and thecore network the transfer technique is the ATM (Asynchronous TransferMode). The ATM transmission technique is a switching and multiplexingsolution particularly relating to a data link layer (i.e. OSI layer 2,hereinafter referred to as an ATM layer. In the ATM data transmissionthe end users data traffic is carried from a source to a destinationthrough virtual connections. Data is transferred over switches of thenetwork in standard-size packets called ATM cells. The ATM cellcomprises a header, the main object of which is to identify a connectionnumber for a sequence of cells forming a virtual channel (VC) for aparticular call through the transport network. A physical layer (i.e.OSI layer 1) may comprise several virtual paths (VP) multiplexed in theATM layer. Each virtual path includes several VCs.

One core network which will use the UMTS radio access network is thegeneral packet radio service (GPRS) which is a new service for the GSMsystem (Global System for Mobile communication), and a similar serviceis also being defined for the 3G mobile systems. A subnetwork comprisesa number of packet data service nodes SN, which in this application willbe referred to as serving GPRS support nodes SGSN (or 3G-SGSNs in the 3Gsystems). As illustrated in FIG. 1, each 3G-SGSN is connected to the RNCin the UTRAN over a transport network so that the 3G-SGSN can provide apacket service for mobile data terminals via several base stations, i.e.cells. The intermediate UTRAN provides a radio access and apacket-switched data transmission between the 3G-SGSN and mobilestations MS. Different sub-networks are, in turn, connected to anexternal data network, e.g. to a public switched data network PSPDN, viaGPRS gateway support nodes GGSN. The GPRS service thus allows to providepacket data transmission between mobile data terminals and external datanetworks when the UTRAN (or the GSM) network functions as a radio accessnetwork.

In order to guarantee interoperability between different vendors ofnetworks and network elements, the present 3G specifications specifythat the RNC and the 3G-SGSN are connected using point-to-point ATMpermanent virtual channel (PVC) connections. These signals can becarried over different transport networks, such as the ATM network orthe Synchronous Digital -Hierarchy (SDH) network. The 3G specificationsdo not, however, specify how these ATM PVC connections are set up butallow the operators and vendors to use different solutions.

A problem in such a system may be fault tolerance of the ATM PVCconnections. In the worst case, a failure in the PVC connections or inthe interfaces at the RNC and the 3G SGSN may block all communication.The current 3GPP lu interface specifications only specify that ifredundancy of pathways of the ATM layer between the CN and the RNC issupported, it shall be implemented using ATM protection switchingaccording to ITU-T recommendation I.630. Since I.630 does not support1:n and m:n architectures another backup PVC connection is required foreach ATM PVC connection, which doubles the number of ATM PVCs betweenRNC and 3G-SGSN.

Most present ATM switches do not support ATM layer protection accordingI.630. This means that the use of the ATM protection between ATM edgeswitch and RNC/3G-SGSN is not necessarily possible. Also the use ofend-to-end ATM layer protection between the RNC and the 3G-SGSN is notpossible, if the ATM network does not support ATM OAM according I.610(i.e. does not generate end-to-end ATM-AIS cells in case of linkfailure).

SUMMARY OF THE INVENTION

An object of the present invention is to provide a new mechanism forproviding fault tolerance.

This and other objects and advantages of the invention are achieved bymeans of communications systems as recited in the attached independentclaim. Preferred embodiments of the invention are described in thedependent claims.

A first aspect of the invention is a communications system comprising

a core network element,

at least one access network element,

a transport network providing Asynchronous Transfer Mode (ATM) permanentvirtual channel (PVC) connections between the core network element andsaid at least one access network element,

two or more ATM interface units in the core network element,

two or more active ATM PVC connections between each of said at least oneaccess network element and said core network element, different ATM PVCconnections from any one of said at least one access network elementbeing connected to different ones of said two or more ATM interfaceunits, and ATM traffic being spread over said different ATM PVCconnections in order to improve tolerance of the communications systemto failures in the ATM PVC connections or in the ATM interface units.

According to the invention, the fault tolerance is achieved by 1)providing two or more active ATM PVC connections between a core networkelement, such as an SGSN, and an access network element, such as an RNC,and 2) providing a separate ATM interface unit for each such ATM PVCconnection in the core network element. As used herein, the term activeconnection refers to a connection carrying user communication orsignalling (in contrast to standby or redundant connections which arekept inactive or in reserve until the primary connection fails). Theuser and signalling traffic is distributed among these activeconnections and interface units. In case of a failure in one of the ATMPVC connections or interface units, the communication is maintained overthe other connection(s) and interface unit(s). Thus, only part of thetransport capacity of the RNC node is lost, and a total block ofcommunication is avoided. Any new communication will be setup over theother connections. In an embodiment of the invention, also the trafficof the lost connection is rerouted via the other connections. Thus,outside of the peak traffic periods, all or major part of the trafficcan be served.

The present invention does not require redundant connections as the ATMprotection switching proposed in the prior art, thus offering savings intransport network costs. Further, in the primary embodiment of theinvention, no special functionality, such as a special protocol, isrequired. Still further, a link or physical layer protection, such asthe ATM or SDH protection switching, is not needed.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be described below in greater detail in connectionwith preferred embodiments, with reference to the accompanying drawings,in which

FIG. 1 illustrates a prior art interconnection of a radio access networkand a third-generation core network over a transport network,

FIG. 2 illustrates a generic protocol stack for the Iu interface,

FIG. 3 shows a communications system having two RNCs connected to the3G-SGSN in accordance with the principles of the present invention, and

FIG. 4 shows a more detailed internal structure of the RNC and 3G-SGSNfor user data and signalling, respectively.

PERFERRED EMBODIMENTS OF THE INVENTION

The present invention can be applied in any telecommunication systemusing ATM PVC connections over a transport network for interconnectingan access network to a core network. The primary field of application ofthe invention is an interconnection between a third-generation mobilenetwork, such as the UMTS, and a third generation core network, such asthe 3G-GPRS. In the following, the preferred embodiments of theinvention will be described by using the UMTS and the 3G-GPRS asexamples.

FIG. 2 illustrates the protocol stack for signalling and user data inthe lu interface. At the bottom of the transport network layer there isan ATM adaptation layer (AAL) which enhances the service provided by theATM layer to support functions required by higher layers. The AALperforms mapping between the ATM layer and the next higher layer. At themoment there are three different types of AAL, namely type 1 AAL (AAL1),type 2 AAL (ML2) and type 5 AAL (AAL5). In FIG. 2, the type 5 AAL (AAL5)is shown. Below the AAL layer an ATM virtual channel (VC) and a virtualpath (VP) are shown. Further below there are SDH layers, such as SDHVC-4 path, SDH multiplex section, SDH Regenerator Section, etc. However,it should be appreciated that other technique may be used instead of theSDH to carry the ATM connections. In the present invention, thetransport network 1 is preferably an ATM network having at least one ATMswitch between the RNC and the SGSN.

FIG. 3 illustrates a communication system having improved faulttolerance in accordance with the present invention. Two access networkelements, such as radio network controllers RNC1 and RNC2, are connectedto a core network element, such as a third generation SGSN. The 3G-SGSNis provided with several ATM interface units. In the embodiment shown inFIG. 3, there are four interface units IU1, IU2, IU3 and IU4. In theRNC1 the user data traffic is distributed over several GTP units GTP1,GTP2, GTP3 and GTP4. The units GTP1 and GTP2 are connected via an ATMswitch 33 to an SDH interface unit 34 and further over the ATM network 1to different interface units IU1 and IU2 by means of dedicated ATMpermanent virtual channel (PVC) connections 31A and 31B, respectively.Similarly, the units GTP3 and GTP are connected through the ATMswitching unit 33 to an SDH interface unit 35 and further over the ATMnetwork 1 to different interface units IU3 and IU4 by means of dedicatedATM PVC connections 31C and 31D, respectively.

The connection between the RNC2 and the 3G-SGSN is based on the sameprinciples. Each of the units GTP1 to GTP6 are connected over dedicatedATM-PVC connections to different interface units 14 in the 3G-SGSN.Thus, each of the interface units IUl-4 may be used by two or more RNCs,so that also the loss of capacity due to a failure in an interface unitin the 3G-SGSN will be distributed over several RNCs.

As a consequence, the user traffic is distributed over four active ATMPVC connections and respective interface units IU. If there is a failurein one of the PVC connections or in one of the interface units IU1-IU4,only twenty-five percent of the capacity of the 3G-SGSN is lost.However, the traffic over the remaining PVC connections and interfaceunits will be maintained. If the remaining capacity allows, the callsfrom the failed connection and interface unit may be rerouted orre-established via the unaffected PVC connections and interface units.

In the GPRS system a call is established by setting up a protocol dataprotocol (PDP) context in the mobile station MS and in the SGSN. The PDPcontext defines different data transmission parameters, such as the PDPtype (e.g. IP), PDP address (e.g. IP address), quality of service QoS,etc. When one of the interface units IU1-IU4 fails (or sometimes alsowhen PVC connections 31A, 31B, 31E and 31F fail), all PDP contextsassociated with the failed unit (or connection) are failed. It ispossible that the failed PDP contexts are left to hang until the mobilestation MS releases them upon detecting that the communication fails. Itis also possible that the 3G-SGSN releases the failed PDP context bysending a release_PDP_context message to the mobile station MS inaccordance with the GPRS specifications. The mobile stations MS whichhave lost their active PDP context can activate them again for theunaffected PVC connections and interface units IU by sending anactivate_PDP_context_request to the 3G-SGSN in accordance with the GPRSspecifications. In case of a failure in the ATM-PVC connection, the PDPcontext of the failed ATM-PVC may be moved (rerouted) to anotherATM-PVC; the 3G-SGSN sends a modify RAB request (i.e.“RAB_Assignment_Request” message of RANAP the protocol) to the RNC inaccordance with the GPRS specifications.

In the preferred embodiment, when the call is rerouted orre-established, it is primarily the 3G-SGSN which decides which of theremaining connections and interface units is selected. The controlfunction, by means of which the most suitable one of the connections isselected, is called a connection Admission Control (CAC) and will bedescribed in more detail below.

A failure in an ATM-PVC connection in the interface unit IUl-4 can bedetected by periodically sending ATM loop-back cells from the RNC overthe ATM PVC connection to the respective interface unit IU in the3G-SGSN. If the connection and the interface unit are operatingproperly, the loop-back cell will be looped back by the 3G-SGSN andreceived at the RNC within a predetermined monitoring period. If theloop-back cells are not received, the RNC considers that the respectiveATM-PVC connection or the respective interface unit IU has failed. The3G-SGSN may also be provided with an internal control system which isable to detect whether the. interface units are operating properly ornot.

FIG. 4 illustrates a more detailed structure of the GTP units, aninterface unit IU and the SDH interface unit 34. Each of the interfaceunits IUl-4 contains for the user data the following protocol stack(from top to bottom): IP, GPT, UDP, IP, ML5, ATM, and SDH. The topmostIP layer is connected to the higher layers (see FIG. 2) via an IPswitching unit 36. In the RNC, each GTP unit 14 contains the followingprotocol stack (from top to bottom): IP, GTP, UDP, IP, ML5, and ATMVC.Each SDH interface unit 34 and 35 contains the protocol layer ATMVP, andSDH. The ATM VC layers from the GTP units are multiplexed to the ATM VPsin the SDH interface unit 34 by means of the ATM switching unit 33. TheATM switching unit 33 allows a flexible cross-connection between the GTPunits and the SDH interface units.

It should be noted that the number of units shown in FIGS. 34 may varyon demand. The minimum requirement is that there is at least two ATMinterface units in the 3G-SGSN.

When the mobile user requests a new connection (e.g. IP connection, UMTSnetwork, has to decide if there is enough free capacity to accept theconnection. This process is called Connection Admission Control (CAC)which is implemented in the 3G-SGSN. In an embodiment of the inventionthe set up of the new IP connection contains following steps: (1) the MSsends a PDP context activation message to the 3G-SGSN, (2) the 3G-SGSNchecks the IP backbone and 3G-SGSN resources, (3) 3G-SGNS selects theGTP unit and sends a RAB_assignment message the to the RNC, (4) the RNCestimates the air interface and radio network resources, (5) the RNCselects a GTP unit and sends a RAB_assignment_complete message to the3G-SGSN. Further, in accordance with the GPRS specifications, the PDPcontext is also created in a gateway support node GGSN.

In accordance with an embodiment of the invention, the CAC functionalityin the 3G-SGSN carries out the following steps: (1) selects an interfaceunit IU during PDP context activation, (2) If needed, triggers a PDPcontext modification in case of a Serving RNC Relocation procedure inorder to place the PDP context into an interface unit IU having enoughcapacity, (3) keeps an equal traffic loading in all interface units IU,(4) estimates the resources of the interface units lUs based on theexisting PDP contexts, configured maximum limits for each IP trafficclass and CPU loading, and (5) estimates the resources of GTP unitsbased on the existing PDP contexts and ATM PVC QoS parameters.

The description only illustrates preferred embodiments of the invention.It is obvious that as technology advances the basic idea of theinvention can be implemented in several different manners. Therefore theinvention and the embodiments thereof are not restricted to the examplesdescribed above, but they may vary within the scope of the claims.

1-9. (canceled)
 10. A core network element in form of serving packetradio support node for a communication system including at least oneaccess network element in form of a radio network controller and atransport network providing Asynchronous Transfer Mode (ATM) permanentvirtual channel (PVC) connections between the core network element andsaid at least one access network element, said core network elementcomprising: two or more ATM interface units; means for providing two ormore active ATM PVC connections between said core network element andeach of said at least one access network element, such that differentATM PVC connections from any one of said at least one access networkelement are arranged to be connected to different ones of said two ormore ATM interface units, and means for spreading an ATM traffic oversaid different ATM PVC connections in order to improve tolerance of thecommunications system to failures in the ATM PVC connections or in theATM interface units.
 11. An access network element in form of a radionetwork controller for a communications system including at least oneaccess network element, a core network element and a transport networkproviding Asynchronous Transfer Mode (ATM) permanent virtual channel(PVC) connections between the core network element and said accessnetwork element, said access network element comprising: means forproviding two or more active ATM PVC connections between said accessnetwork element and two or more ATM interface units of said core networkelement, such that different ATM PVC connections from said at least oneaccess network element are arranged to be connected to different ones ofsaid two or more ATM interface units of said core network element, andmeans for spreading an ATM traffic over said different ATM PVCconnections in order to improve tolerance of the communications systemto failures in the ATM PVC connections or in the ATM interface units.